There exists a battery pack of a secondary battery using a lithium ion battery or the like configured by plural secondary batteries (hereinafter, referred to as “cell”) connected in series. For such a battery pack, when the voltage difference between cells becomes large by repeating charging and discharging, self-discharge or the like, the capacity balance between the cells (hereinafter, referred to as “cell balance”) may not maintained. When the cell balance is not maintained, the charging capacity capable of being used as a battery pack is lowered to decrease efficiency of charging and discharging.
Further, when charging and discharging are repeated while the cell balance is not maintained, the voltage difference between cells becomes larger so that there may be a case that the cell having the voltage near the overcharge voltage and the cell having the voltage near the over-discharge voltage are provided in the battery pack. Thus, a situation occurs that the battery pack cannot be charged or discharged even though the individual cells are not deteriorated.
Therefore, a function for maintaining the cell balance is necessary for the battery pack including plural cells.
FIG. 1 is a diagram for explaining a conventional charge-discharge control circuit. A conventional charge-discharge control circuit 50 controls charging and discharging of a battery pack by controlling ON/OFF of a transistor for controlling charging MDOUT that controls charging of the battery pack and a transistor for controlling discharging MDOUT that controls discharging from the battery pack.
In the battery pack, cells BAT1 to BAT3 connected in series and a voltage detection circuit 70 that detects the voltage of the cells BAT1 to BAT3 are provided. The voltage detection circuit 70 is composed of voltage detection circuits 10, 30 and 35 respectively provided for the cells BAT1 to BAT3. The voltage detection circuits 10, 30 and 35 have the same structure. The voltage detection circuits 10, 30 and 35 detect that the voltages of the cells BAT1 to BAT3 reach an overcharge detection voltage or an over-discharge detection voltage, respectively.
When one of the cells BAT1 to BAT3 is detected to reach the overcharge detection voltage, a charge terminate signal indicating to terminate charging of the battery pack is output from the NOR circuit 18 via a level shift circuit 19. By the charge terminate signal, the transistor for controlling charging MCOUT is switched off to terminate charging of the battery pack.
Further, when one of the cells BAT1 to BAT3 is detected to reach the over-discharge detection voltage, a discharge terminate signal indicating to terminate discharging from the battery pack is output from an AND circuit 17. By the discharge terminate signal, the transistor for controlling discharging MDOUT is switched off to terminate discharging from the battery pack.
Further, for the cells BAT1 to BAT3, bypass circuits 20, 40 and 60 are respectively provided to be in parallel with the corresponding cells BAT1 to BAT3. The bypass circuits 20, 40 and 60 have the same structure. The bypass circuit 20 is composed of a resistor Ra and a switch element 21 which are connected in series. The charge-discharge control circuit 50 controls the switch element 21 of the bypass circuit 20 based on the voltages of the cells BAT1 to BAT3 detected by the voltage detection circuits 10, 30 and 35 to decrease the voltage difference between cells.
FIG. 2 is a diagram showing the voltage detection circuit. The voltage detection circuit 10 corresponds to the cell BAT1. As the voltage detection circuits 30 and 35 have the same structure as the voltage detection circuit 10, explanation is not repeated.
The voltage detection circuit 10 includes a comparator 11, a charge terminate delay circuit 12, a comparator 13, a cell balance delay circuit 14, a comparator 15, and an over-discharge delay circuit 16. The bypass circuit 20 is connected with the cell BAT1 in parallel.
The comparator 11 detects the termination of charging of the cell BAT1. The charge terminate delay circuit 12 outputs a charge terminate signal that terminates charging of the battery pack after being delayed for a predetermined period when the overcharge is detected by the comparator 11. The comparator 13 detects that the voltage of the cell BAT1 reaches a cell balance detection voltage VEAL, and detects that the voltage of the cell BAT1 is lowered to a cell balance detection voltage VBAU. The cell balance delay circuit 14 outputs a control signal that causes the switch element 21 to be switched on after being delayed for a predetermined period when the voltage of the cell BAT1 becomes the cell balance detection voltage VBAL. The over-discharge delay circuit 16 outputs a discharge terminate signal that terminates discharging from the battery pack after being delayed for a predetermined period when the comparator 15 detects the over-discharge. The overcharge detection voltage, the cell balance detection voltage and the over-discharge detection voltage are set by resistors R.
When the switch element 21 is switched on in the bypass circuit 20, the current is divided so that the charge current that flows through the cell BAT1 is reduced. Similarly for the other cells BAT2 and BAT3, when each of the cells BAT2 and BAT3 reaches the cell balance detection voltage, the current is divided to reduce the charge current that flows through the corresponding cell BAT2 or BAT3 to decrease the voltage difference between cells.
In addition to the example shown in FIG. 2, Patent Document 1, for example, discloses a cell voltage equalizing apparatus for a battery pack capable of performing equalization even when the difference between the voltages of the cells is large. Further, Patent Document 2 discloses a method capable of appropriately and safely controlling charging of secondary batteries connected in series to prevent shortening of the lifetime of the batteries. Further, Patent Document 3 discloses a method in which a cell whose voltage is higher is detected among plural cells and, the detected cell is discharged by a discharging circuit provided in the battery pack to equalize the cell balance.